Say nothing---for a variety of reasons.
1st: what, exactly, do you have by way of 'proof' that she has or hasn't had sex in that time period?
2nd: even if she is lying, you'd be acting like a moron confronting her on a 3rd Grade level ('You lied to me!! You told a lie!!') Your interpretation, IMO, is probably correct---but confronting her will make her defensive and embarrassed and probably drive her off. And she will likely protect herself against you and get even by turning her friends against you...negative social proof, who needs it?
Better: figure out what she was attempting to do by that lie:
Was she trying to put up a front and show she was more sophisticated and worldly? Was she trying to project that she wasn't a "can't get a date" type loser? Was she engaged in one-upmanship? This is far more productive for you to figure out than trying to cover a mistake.
3rd: you made a mistake getting pissed and she knows it too. Or maybe not. Covering it up or trying to re-work it later will only make it more obvious and keep it going longer. She's probably already well on her way towards forgetting it--like old news.
And why were you answering a retarded question like that in the first place (when did you last have sex)--??? Just say "that's classified" and move on (or some other variation.) You're not obligated to answer every question a girl asks you. Don't kiss and tell---no bragging.